This invention relates generally to gas fired furnaces and, more particularly, to a method and apparatus for controlling a gas furnace having a gas valve which is susceptible to being stuck in the open position.
Furnaces may be either of the natural draft or the power draft types. In the power draft furnace, a motor driven blower is operated to motively draw (induced) or blow (forced) the combustion air to the burner to thereby enhance the combustion process. A normal sequence of operation when the thermostat calls for heat is for the inducer motor to come on to purge the system of any gases that might be present. An ignitor is then turned on and a gas valve is opened to initiate the combustion process. A flame sensor circuit then operates to ensure that the burner is operating properly, and then the circulating air blower is turned on to force the heated air into the room. When the room is heated to the point where the thermostat setting is satisfied, the thermostat is turned off, and the gas valve and inducer motor are turned off. After a predetermined delay, the blower is then turned off.
In existing systems, if the gas valve should stick in the open position during a heating cycle and thereby remain open when the room thermostat turns off, the gas will continue to flow and remain ignited even though the inducer motor will be turned off as a function of the normal sequence. Without combustion air being supplied by the inducer, the combustion process will be inhibited and a build up of gas will result. This may in turn cause an undesirable flame roll out with possible resulting damage to the furnace.
It is therefore an object of the present invention to provide an improved control system for an induced, gas fired furnace.
Another object of the present invention is the provision in a gas furnace for reducing the occurrence of flame roll outs.
Yet another object of the present invention is the provision in a gas furnace having a gas valve which is susceptible to sticking in the open position, for reducing the occurrence of flame roll outs.
Still another object of the present invention is the provision in a gas furnace for reducing the occurrence of gas build up when the gas valve sticks in the open position.
Another object of the present invention is the provision in an induced draft, gas furnace for a control system which is economical to manufacture and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.